Phase Transformations in the CeO₂–Sm₂O₃ System: A Multiscale Powder Diffraction Investigation

Abstract

The structure evolution in the CeO-SmO system is revisited by combining high resolution synchrotron powder diffraction with pair distribution function (PDF) to inquire about local, mesoscopic, and average structure. The CeO fluorite structure undergoes two phase transformations by Sm doping, first to a cubic (C-type) and then to a monoclinic (B-type) phase. Whereas the C to B-phase separation occurs completely and on a long-range scale, no miscibility gap is detected between fluorite and C-type phases. The transformation rather occurs by growth of C-type nanodomains embedded in the fluorite matrix, without any long-range phase separation. A side effect of this mechanism is the ordering of the oxygen vacancies, which is detrimental for the application of doped ceria as an electrolyte in fuel cells. The results are discussed in the framework of other Y and Gd dopants, and the relationship between nanostructuring and the above equilibria is also investigated.

Local and Average Structure of Yb-Doped Ceria through Synchrotron and Neutron Pair Distribution Function

“…The two-phase model significantly improves the fit, especially in the region featuring M-M distances. Similar results were already observed in ceria doped with Y [23,35], Sm [25], Gd [24,39], Bi [40], Zr [41], Pr, Tb [16], and La [42]. This is also consistent with recent absorption spectroscopy study, where a bimodal distribution of bond lengths was observed in Y-and Sm-doped ceria compounds [43].…”

“…The (413) reflection is already observed for Yb375, even though with small intensity and large broadening, about four times broader than structure peaks. This phenomenon, consistent with the existence of anti-phase boundaries due to the nucleation and growth of C-type nanodomains starting on different sites, is consistent with the hierarchical structures already observed for Y, Gd, and Sm and described as C* [23][24][25]. Lastly, x(M2) is very close to 0 (−0.003), indicating that only a slight C-type distortion occurs.…”

“…As to lower dopant concentrations, the small magnitude of the peak at 4.1 Å, which is comparable to the nearby ripple, hinders the correct estimation of the C-type fraction. However, in view of the progressive evolution of msd and according to previous investigations on other dopants [23,25,39], we believe that C-type ordering occurs already for smaller concentrations.…”

“…Structural evolution linked with heavy doping is an important factor to understand and to estimate the way defects interact at lower dopant concentration. Indeed, it was proved recently by PDF that for Y [23], Gd [22,24], and Sm [25], the fluorite to C-type transformation occurs via nucleation and growth of distorted C-type nanodomains that merge and percolate at a critical dopant concentration. This makes heavily doped ceria structure a hybrid between those of fluorite and C-type [5,26].…”

In Situ High Pressure Structural Investigation of Sm-Doped Ceria

“…Ionic conductivity is strictly related to structural and microstructural issues, and this is the reason why so many papers in recent years have been devoted to the crystallographic properties of this material, even in the light of the non-negligible effects of extrinsic factors (e.g., thermal treatments) on the stability of the various oxide phases [15,16]. A particular attention has been paid to defect aggregations [3,[17][18][19][20][21], since ionic conductivity in doped ceria occurs through the hopping of oxygen ions towards the vacancies (V .. O ) created for charge compensation when Ce 4+ is partially substituted by RE 3+ . The mechanism works until the fluorite-type cubic structure of CeO 2 (hereafter named F; space group: Fm3m; Ce coordination number: 8) is retained (i.e., until the solid solution of RE 3+ within the CeO 2 matrix is stable).…”